One reason though is that he campaigns in the lower division and the money is not as big as lets say in the premier weight which is 147 lbs. But I read that Donaire and his wife has a good investment with the money that Donaire earn in boxing, so maybe they will have enough money even if Donaire decided to retire for good. So still a win win situation for him, if by chance he lost and decided to quit, he will have a good life. And if we wins, then obviously he will fight another day to defend that belt and make a payday for him.
It's very important for a boxer to learn how to manage their finances. Donaire is lucky to have his wife as his manager, as the money will not go to someone else. With good investments, their wealth has likely grown and they have already prepared for their future, even without the money they are earning from boxing.
So, he may retire as a champion if that's his goal. However, if he still has a passion for boxing and believes in himself that he can win, he might choose to stay in the sport for longer.